Note :- Althogh childByAutoId() is a great functionality from Firebase.But prefer timestamps to store the data into when you want to create a node with a unique key.The reason why i prefer timestamps is because they can also be helpful in sorting of data...But thats just me..

Alternative :-

let timeStamp = Int(NSDate.timeIntervalSinceReferenceDate()*1000) //Will give you a unique id every second or even millisecond if you want..
FIRDatabase.database().reference().child("Posts").child(timeStamp).setValue(postInfo)